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When an aircraft flies into the vicinity of one of the magnetic poles, why does the magnetic compass becomes unreliable or even useless?
  • A
    The Horizontal component becomes so weak that the directive force is insufficient for a reliable compass indication.
  • B
    The influence of the abundant local magnetic anomalies caused by ionised particles in the vicinity of the magnetic poles.
  • C
    The influence of Aurora Borealis (northern Lights) tends to deflect the compass.
  • D
    The decrease of the earth's magnetic field strength causes the directive force to be not sufficient for a reliable compass indication.

Refer to figure. 
A freely suspended magnet will align itself with the Earth's magnetic field. The total field (T) is resolved into a horizontal component (H) and a vertical component (Z). 

HORIZONTAL COMPONENT
The horizontal component is the part which is detected by the compass needle in order to determine Magnetic North. A compass is freely suspended in the horizontal plane and the needle will align itself with the Earth's horizontal component (H) of the Earth's magnetic field.

  • The compass is ineffective near the poles. As you approach the poles, the angle between magnetic flux and the horizon becomes vertical – compass ‘dip’ becomes a factor => decrease in horizontal magnetic field, increase in vertical magnetic field.
  • The value of the horizontal component is maximum at the Magnetic Equator, a freely suspended compass needle in the Earth's magnetic field will be horizontal to the Earth - in line with the horizontal component. 

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